1. Field of the Invention
The present invention relates to a receiving processing method and a receiving apparatus. The invention relates particularly to an art which is suitable for use in communication systems which employ the multipath interference countermeasure technology, so-called guard interval, such as the OFDM (Orthogonal Frequency Division Multiplexing) access method.
2. Description of the Related Art
OFDM is considered to be a promising access method in the next-generation (fourth generation) mobile communication system. OFDM has recently been employed in communication applications which realize networks such as digital terrestrial television broadcasting, ADSL (Asynchronous Digital Subscriber Line), and high-speed wireless LAN (Local Area Network).
OFDM uses different subcarriers (propagation waves) each having a unique center frequency, thereby realizing a high frequency efficiency. That is, transmission data is subjected to serial/parallel (S/P) conversion to divide the data into smaller segments, and modulation is performed to add the segments onto different subcarriers (sub-propagation waves) which have been complex modulated into I and Q components, and inverse fast Fourier transformation (IFFT) processing is performed to transmit the data in parallel. By means of dividing the data as described above, the symbol transmission rate per propagation wave can be reduced in comparison with serial transmission.
As a result, overall influence of fading (a phenomenon in which the intensity of a signal in radio communication greatly varies time-wise and space-wise) is reduced. In addition, upon transmission, each sub-propagation wave band is “orthogonal” with every other sub-propagation wave band, so that adjacent propagation wave bands will not interfere with each other even if they are so close that they overlap.
Further, for the purpose of providing an improved immunity to multipath interference, OFDM normally uses a “guard interval (GI)” which is a part of data sent redundantly in time, so that if signals (ghost) deviating in time, due to diffuse reflection or the like, arrive at the receiver position, it is possible to provide a system resistant to multipath interference.
A guard interval (GI) is a copy of the later half (last) portion of the valid symbol 100 appended to the front of the symbol, as shown in FIG. 18. This means that the length of one OFDM symbol is lengthened by the length of the guard interval 101, taking the delay times of multipath waves (delay waves) into consideration.
Here, the OFDM signal receiver end performs the processing inverse of that performed by the transmitter end. More specifically, the guard interval 101 is removed from the received signal, and different subcarrier signals are reproduced by Fast Fourier Transformation (FFT), and the received data is demodulated, and parallel/serial (P/S) conversion is performed to obtain data as the received signal.
As shown in FIG. 19, use of the guard interval 101 makes it possible to accommodate only the valid symbol portion 100 in the FFT interval (FFT subject time scope) when the amounts of delay of delay waves are equal or smaller than the guard interval length (period) (see delay wave 1 and delay wave 2), so that the deterioration of the characteristic caused by the delay waves 1 and 2 is greatly reduced (can be substantially ignored). Accordingly, an increased guard interval length will make possible the avoidance of multipath interference even when the amount of delay of delay waves included in the received signal of a mobile terminal is great.
However, if a delay wave exceeds the guard interval length (see delay wave 3), a component of the preceding symbol is mixed into the object symbol, thereby causing inter-symbol interference, so that reception characteristics are deteriorated. In addition, as the cell radius becomes increasingly greater, the amount of delay of a delay wave also becomes greater, resulting from an increase in the signal propagation distance.
Consequently, as the cell radius becomes increasingly greater, the length of the guard interval to be inserted into the OFDM signal transmitted over the radio zone of the cell needs to be increased. However, since guard intervals 101 are used as a countermeasure against delay waves, guard intervals 101 which are likely to be influenced by delay waves will not be subjected to the reception FFT, and are removed from the received signal as already described above (see FIG. 20).
Here, in cellular systems, an environment where the cell radius is several tens of km can exist. Thus, in a case where the length of a guard interval 101 is set to a distance to the edge of the cell or to a distance in conformity to the above, since a delay wave is short in the vicinity of the cell, if the guard interval 101 is simply removed, the remaining long guard interval 101, which is not affected by the delay waves, becomes useless, thereby causing deterioration in transmission efficiency.
Hence, as proposed in the following patent document 1, as shown in FIG. 21, a portion 1010 of the guard interval 101 which is not influenced by a delay wave (which does not interfere with another symbol) is found out, and only this portion 1010 is combined with the received symbol (valid symbol portion 100). In this manner, effective use of the portion 1010 of the guard interval 101, which portion does not interfere with another symbol, makes possible good reception even when a level variation due to e.g., fading occurs.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2000-151542
However, since the above-mentioned previous art needs a special detecting means (inter-symbol interference detecting circuit) for detecting the portion 1010 which does not interfere with another symbol, it is accompanied by an issue of an increase in the size of the apparatus and the cost. Further, since the art of the above patent document 1 does not take the transmission condition (modulation scheme and/or coding rate) of the transmission data into consideration, it cannot necessarily be said that the optimum reception characteristic is obtained.